Are you certain the files being downloaded are MP3's? and not WMA's, AIFF's, Midi files or any other file type?
If you would like me to test this scenario for you in our lab and check your current configuration, could you make a new ticket on the helpdesk and include the following information with it. Thanks.
http://support.qbik.com/index.php?_a=tickets&_m=submit
1. WinGate Registry
GateKeeper --> Options menu --> Advanced --> Save Registry
Or if you can't access the Gatekeeper,
Start --> Programs --> WinGate --> Advanced Options --> Registry --> Export Settings
2. WinGate Config Report
GateKeeper --> Options menu --> Advanced --> Save Config Report
3. ipconfig/all from one LAN Client
(Windows) Start menu --> Run --> cmd --> ipconfig/all >> C:\ipa-client.txt
4. ipconfig/all from the WinGate Server
(Windows) Start menu --> Run --> cmd --> ipconfig/all >> C:\ipa-server.txt